1. Field of the Invention
The present invention relates to an industrial robot. In particular, the present invention relates to a horizontal articulated robot of the full-earth type.
2. Description of Related Art
In general, a horizontal articulated robot comprises a base pillar, a first arm which is installed in such a way that it can rotate over a horizontal plane with a longitudinal direction end of the first arm taken as a center of rotation and the base pillar serving as a center axis, and a second arm. The second arm is installed in such a way that it can rotate over a horizontal plane with its longitudinal direction end taken as a center of rotation and the other longitudinal direction end of the first arm serving as a rotational center axis. A driving mechanism that serves as a unit for driving the second arm is generally fixed to either the second arm itself or the first arm.
In this case, the driving of the second arm by the second arm driving mechanism is affected by the movement of the first arm, giving rise to a problem that acceleration and deceleration cannot be controlled independently of each other.
As a means for solving this problem, a full-earth type robot has been proposed. The full-earth type robot is disclosed in Japanese Patent application Nos. 06-68192 and 06-64339 with their corresponding patent application Ser. No. 08/396,719 filed in the U.S. on Mar. 1, 1995.
In the case of a full-earth type robot, the second arm driving unit for driving the second arm is fixed to neither the first nor second arm. Instead, such a robot has a configuration wherein the second arm driving unit is fixed to a base in an attempt to reduce the influence of the first arm on the movement of the second arm.
In addition, the full-earth type robot described above in general has an RZ shaft at the end of the second arm. The RZ shaft is also known as a working shaft typically used for installing an RZ tool. The RZ tool expands and contracts in the vertical (or Z) direction and, at the same time, rotates itself in the R direction. Normally, a driving mechanism for driving the RZ shaft is fixed on the second arm.
In the case of the full-earth type robot described above, however, the entire driving mechanism of the RZ shaft is installed on the second arm. Accordingly, in order for the position angle of the R axis of the second arm to sustain an absolutely constant positional relation regardless of the positions of the first and second arms, control must be exercised to correct the position angle of the R axis depending upon the positions of the first and second arms.
Such control thus becomes complicated and, on top of that, the direction of the R axis may vary during its movement accompanying changes in position of the first and second arms due to, among other causes, a lag occurring in the servo control. As a result, inconvenience is encountered in carrying out the precision control of the R axis.